1. Technical Field
The present invention relates to a high frequency filter employing helical resonators, and more specifically, to adjustment of the coupling opening or openings of filters employing helical resonators.
2. Discussion of the Relevant Art
The use of a helical resonator as a circuit element is well known in the art, and is widely used in filters of a high frequency range, in particular 100 to 1000 MHz. Such resonators comprise elements which are a helically wound coil and a metallic cover surrounding said coil at a distance. The low-impedance (grounded) end of the coil may be directly connected to the metal cover. In practice, this takes place in that a wire to be wound into a helical coil is at this end straight for some length and positioned so as to be approximately perpendicular to the end face of the resonator cover, whereby a first turn of the helical coil is at a length of the straight leg from the end face of the cover. The opposite, high-impedance end of the coil is in the proximity of the cover, being capacitively coupled thereto. The resonator can be connected electrically to the rest of the filter circuit either so that the low-impedance end is not connected to the cover; instead, a connecting lead insulated from the cover is connected thereto, or at a certain point of the helical coil is soldered a connecting lead which, being insulated from the cover, is taken outside the cover. The resonant frequency of the helical resonator is the function of the physical dimensions of the coil, the capacitive structure, and the distance between the high-impedance end of the coil and the cover. Therefore, for obtaining a resonator of a given frequency range, an accurate and exact construction is required for manufacturing.
From the Finnish patent No. 78198 is known a helix resonator in which the helical coil has been supported with an insulating plate, whereby in one part of the insulating plate is positioned an electrical circuit formed from micro strips, to which the resonator has been electrically connected. The procedure of how to produce a helix resonator which is accurate concerning its tapping point and reproducible is described in the Finnish patent application No. 884953. The construction disclosed therein is partly the same as in the resonator disclosed in the Finnish patent No. 78198, with the exception that the micro strip is positioned at a given point of the surface of the insulating plate, whereby, when a coil is inserted to the insulating plate, it is always coupled to the same point of the micro strip. The micro strip can be taken out from the resonator directly or it may be connected to the electric circuit of an insulating plate disclosed in the Finnish patent No. 78198, the plate acting as a support.
Such high frequency filters employing helix resonators are known in the art which comprise a metallic or metallized cover housing a number of helix-shaped resonator coils separated from each other by metallic or metallized partitions, wherein coupling apertures have been made for regulating the electrical coupling between the separate resonators. The coupling aperture is simply an aperture of a given size punched in the wall between the resonators. In different filter versions the aperture size is different for different resonant frequencies, that is, each version has a specific aperture size. The size of the aperture has to be highly precise, the tolerance for its width and height being +/- 0.01 mm in practice. Therefore, a specific punching tool has heretofore been provided for each aperture of a given size, that is, there is a punching tool for each aperture size. One of the drawbacks of this technique is that a great number of tools are needed, namely, as many tools as there are aperture sizes, and considering the high price of such tools, the technique has a cost-increasing effect. Another drawback is that the dimensioning differences of the apertures are sometimes very small indeed, whence follows the risk that covers of similar appearance, but with slightly different apertures become mixed up. One more cost-increasing drawback is the need of large intermediate stores in large-scale serial production.